1. Field of the Invention
This invention relates to error reduction in communications systems, and more particularly to receiver-based error reduction in systems having variable signal channel quality.
2. Description of the Prior Art
In the continuing search for more reliable communications systems, many different approaches have been tried. One very useful approach has been to make use of a process of sampling and quantizing, which converts continuous scales of time and amplitude, respectively, into discrete scales, permitting representation of the signal by a finite number of quantized coded symbols. As described in U.S. Pat. No. 2,681,385, issued to B. M. Oliver on June 15, 1954, sampling alone does not entail any loss of information if the sampling frequency is at least twice as great as the highest frequency of interest in the information signal. Furthermore, quantization does not cause a serious loss of information if the number of quantizing levels is sufficiently high.
The quantized coded signals are normally transmitted across an information channel, i.e., from transmitter to receiver, in binary coded form. Such a coded signal consists of a timewise serial train of pulses and spaces representing "high" (or "1") and "low" (or "0") states, respectively. Each pulse and space constitutes a "bit" of the binary code, and n bits in time sequence constitutes an n bit "word". Thus, for example, a "three-bit" binary coded signal consists of a sequence of "three-bit" words, each of which represents a quantum level of the sampled and quantized input analog signal.
An error in the state of one of the binary word bits can cause a very large error in the resulting output quantum level. For example, a transmission error in the leftmost bit of the transmitted binary word 1001 representing the decimal quantum value 9 would result in the word being received as the binary word 0001 representing the decimal quantum value 1. Single bit errors in received binary coded signals can thus cause serious distortion of the receiver output analog signal.
Much progress has been made in bit error detection and correction apparatus for quantized coded information transmission systems. The common approaches used include: (1) repetition of data techniques; (2) transmission over multiple channels; and (3) parity checking with error detecting codes. Each of these approaches requires additional equipment at both the transmitter and receiver. Examples of such systems are found in U.S. Pat. No. 3,444,516, issued to Joseph W. Lechleider on May 13, 1969, and U.S. Pat. No. 3,475,723, issued to Herbert O. Burton on Oct. 28, 1969.
Solely receiver-located error detection and correction is desirable, since it avoids the need for more complex transmitters. The article "Average and Median Based Smoothing Techniques for Improving Digital Speech Quality in the Presence of Transmission Errors," IEEE Transactions on Communications, Volume 24, pages 1043-1045, September, 1976, discloses receiver-located smoothing techniques for noise attenuation in signals from digital channels with relatively high error probability.
The article "Detection and Partial Correction of Isolated Errors from the Received Data in a 1st-Order D.P.C.M. CODEC," by R. Steele and M. A. Yeoman, Electronics Letters, Volume 11, No. 11, pages 230-232, May, 1975, discloses a slope-limited transmission and reception system which senses and partially modifies high divergent data points in a received signal based on its deviation from the known slope limitations of the transmitted signal. In this slope limited system, however, the analog signal must be sampled appreciably above the Nyquist rate. When a highly divergent data point is detected, the deviant sample is replaced by the value of the most recent preceding reconstructed sample which does not violate the slope limitations of the transmitted signal. The reconstructed samples constitute the output of a slope-limited differential pulse code modulator located in the receiver.
It is an object of the present invention to provide an improved arrangement for receiver-located error detection and correction wherein a received signal sampled at or above the Nyquist rate is selectively corrected by a smoothing filter only when apparent errors have been detected.